Space

Sharper images of a supermassive black hole reveal new secrets

Sharper images of a supermassi...
An artist's rendition of a supermassive black hole, such as the one at the center of the Milky Way
An artist's rendition of a supermassive black hole, such as the one at the center of the Milky Way
View 3 Images
Top left: a simulated black hole with no scattering. Top right: a simulated black hole with scattering effects. Bottom right: How Sagittarius A* appears in the sky, naturally scattered. Bottom left: The processed image of the black hole, with the scattering removed
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Top left: a simulated black hole with no scattering. Top right: a simulated black hole with scattering effects. Bottom right: How Sagittarius A* appears in the sky, naturally scattered. Bottom left: The processed image of the black hole, with the scattering removed
An artist's rendition of a supermassive black hole, such as the one at the center of the Milky Way
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An artist's rendition of a supermassive black hole, such as the one at the center of the Milky Way
A diagram showing where the various telescopes in the GMVA are located around the Earth
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A diagram showing where the various telescopes in the GMVA are located around the Earth

Studying black holes is a tricky business, but not always because they're so dim – to the contrary, in some cases it's hard to see through the bright cloud of hot gas that surrounds them. Now, a team of astronomers has used a global array of telescopes to peer through that fog and capture some of the clearest radio images yet of Sagittarius A*, the supermassive black hole at the center of the Milky Way. And it may indicate that the object just happens to be pointing directly at Earth.

About 26,000 light years away, Sagittarius A* is probably not the closest black hole to Earth, but it is the closest supermassive one, with the mass of around 4 million Suns. That makes it the ideal target for astronomers looking to snap an image of a black hole. Of course, since light itself can't escape the object's gravitational pull, there's not much to see on its own, but the goal is to see the "black hole shadow" – the silhouette of the object framed against the bright background of matter falling into the black hole.

But the problem is that since there's half a galaxy between it and us, the light is scattered by all the matter in our line of sight, creating a bright cloud that obscures Sagittarius A*. To see through that, astronomers have now essentially used a virtual telescope the size of Earth.

This technique is known as very-long-baseline interferometry (VLBI). It's performed by pointing several different radio telescopes at the same object at the same time, then comparing the differences between when the signals from the source hit each of the telescopes. That data can then be processed to remove most of the scattering effect, producing a clearer radio image of the object.

Astronomers used the Global Millimeter VLBI Array (GMVA) to do just that, observing Sagittarius A* at a frequency of 86 GHz. But the key development is that this was the first time the Atacama Large Millimeter Array (ALMA) telescope in Chile was used as one of 13 telescopes in the GMVA, which is particularly sensitive at this frequency. The end result was an image with twice the resolution of previous attempts at 86 GHz.

Top left: a simulated black hole with no scattering. Top right: a simulated black hole with scattering effects. Bottom right: How Sagittarius A* appears in the sky, naturally scattered. Bottom left: The processed image of the black hole, with the scattering removed
Top left: a simulated black hole with no scattering. Top right: a simulated black hole with scattering effects. Bottom right: How Sagittarius A* appears in the sky, naturally scattered. Bottom left: The processed image of the black hole, with the scattering removed

The study revealed a few new details about the supermassive black hole. The source of the radio emissions was found to be symmetrical, and most of the signals are coming from one area much smaller than previously thought – covering just one 300 millionth of a degree in the sky. That surprise may change our understanding of what exactly is giving off the signals – or that Earth has lucked into a very specific position to view it from.

"This may indicate that the radio emission is produced in a disk of infalling gas rather than by a radio jet," says Sara Issaoun, lead researcher on the project. "However, that would make Sagittarius A* an exception compared to other radio-emitting black holes. The alternative could be that the radio jet is pointing almost at us."

In future, an even more sensitive telescope array, known as the Event Horizon Telescope, is planning to study Sagittarius A* at a frequency of 230 GHz. That should return even higher resolution images of the supermassive black hole.

"Even though scattering blurs and distorts the image of Sagittarius A*, the incredible resolution of these observations allowed us to pin down the exact properties of the scattering," says Michael Johnson, co-author of the study. "We could then remove most of the effects from scattering and begin to see what things look like near the black hole. The great news is that these observations show that scattering will not prevent the Event Horizon Telescope from seeing a black hole shadow at 230 GHz, if there's one to be seen."

The research was published in The Astrophysical Journal.

Source: Netherlands Research School for Astronomy via Phys.org

6 comments
juanhollisDS4E
It is amazing that these so called "black holes" are still thought of as a hole! BUT in fact they are Dark Matter Spheres and work the same way as a star system, by holding the entire Galactic system together otherwise if was not for D M Sphere's Galaxies would scatter into the void of space due to the enormous rotational speed of the galaxies! Also IF it was a black hole and nothing can escape it then all the galaxies would be swallowed up by this so called "black hole" which is not the case.
F. Tuijn
The possibility of the jet pointing as Earth reminds me of Fred Hoyle's novel Inferno.
guzmanchinky
These are astonishing phenomena. Just their existence makes me realize we are just taking the first baby steps of science.
Grunchy
I bet a real black hole would be quite loud to be around. I bet it's easily over 100 dB.
warren52nz
@Grunchy. There's no sound in space....
amazed W1
Somebody put me right, but if light is bent by gravitational fields, then a concentrated mass of 4 million suns would not allow a shadow to appear because the light that isn't swallowed up by it, including that which just escapes, would be bent around the mass and so mask the hole. This assuming energy and mass are merely different states of the same thing.